Install thread profiling/tracing hooks in ThreadPool worker threads while the task runs.

Fixes #1670

docs/changes/1678.feature

+Make worker threads created by :class:`gevent.threadpool.ThreadPool` install
+the :func:`threading.setprofile` and :func:`threading.settrace` hooks
+while tasks are running. This provides visibility to profiling and
+tracing tools like yappi.
+
+Reported by Suhail Muhammed.

src/gevent/tests/test__threadpool.py

@@ -721,5 +721,92 @@ class TestThreadResult(greentest.TestCase):
         self.assertIsNotNone(tr.receiver)
 
 
+class TestWorkerProfileAndTrace(TestCase):
+    # Worker threads should execute the test and trace functions.
+    # (When running the user code.)
+    # https://github.com/gevent/gevent/issues/1670
+
+    old_profile = None
+    old_trace = None
+
+    def setUp(self):
+        super(TestWorkerProfileAndTrace, self).setUp()
+        self.old_profile = gevent.threadpool._get_thread_profile()
+        self.old_trace = gevent.threadpool._get_thread_trace()
+
+    def tearDown(self):
+        import threading
+        threading.setprofile(self.old_profile)
+        threading.settrace(self.old_trace)
+
+    def test_get_profile(self):
+        import threading
+        threading.setprofile(self)
+        self.assertIs(gevent.threadpool._get_thread_profile(), self)
+
+    def test_get_trace(self):
+        import threading
+        threading.settrace(self)
+        self.assertIs(gevent.threadpool._get_thread_trace(), self)
+
+    def _test_func_called_in_task(self, func):
+        import threading
+        import sys
+
+        setter = getattr(threading, 'set' + func)
+        getter = getattr(sys, 'get' + func)
+
+        called = [0]
+        def callback(*_args):
+            called[0] += 1
+
+        def task():
+            test.assertIsNotNone(getter)
+            return 1701
+
+
+        before_task = []
+        after_task = []
+
+        test = self
+        class Pool(ThreadPool):
+            class _WorkerGreenlet(ThreadPool._WorkerGreenlet):
+                # pylint:disable=signature-differs
+                def _before_run_task(self, func, *args):
+                    before_task.append(func)
+                    before_task.append(getter())
+                    ThreadPool._WorkerGreenlet._before_run_task(self, func, *args)
+                    before_task.append(getter())
+
+                def _after_run_task(self, func, *args):
+                    after_task.append(func)
+                    after_task.append(getter())
+                    ThreadPool._WorkerGreenlet._after_run_task(self, func, *args)
+                    after_task.append(getter())
+
+        self.ClassUnderTest = Pool
+
+        pool = self._makeOne(1)
+        assert isinstance(pool, Pool)
+
+        setter(callback)
+
+
+        res = pool.apply(task)
+        self.assertEqual(res, 1701)
+        self.assertGreaterEqual(called[0], 1)
+
+        # The function is active only for the scope of the function
+        self.assertEqual(before_task, [task, None, callback])
+        self.assertEqual(after_task, [task, callback, None])
+
+    def test_profile_called_in_task(self):
+        self._test_func_called_in_task('profile')
+
+    def test_trace_called_in_task(self):
+        self._test_func_called_in_task('trace')
+
+
+
 if __name__ == '__main__':
     greentest.main()

src/gevent/threadpool.py

@@ -40,6 +40,17 @@ def _format_hub(hub):
         hub.__class__.__name__, id(hub), hub.thread_ident
     )
 
+
+def _get_thread_profile(_sys=sys):
+    if 'threading' in _sys.modules:
+        return _sys.modules['threading']._profile_hook
+
+
+def _get_thread_trace(_sys=sys):
+    if 'threading' in _sys.modules:
+        return _sys.modules['threading']._trace_hook
+
+
 class _WorkerGreenlet(RawGreenlet):
     # Exists to produce a more useful repr for worker pool
     # threads/greenlets, and manage the communication of the worker
@@ -137,12 +148,27 @@ class _WorkerGreenlet(RawGreenlet):
                   file=stderr)
             tb = tb.tb_next
 
+    def _before_run_task(self, func, args, kwargs, thread_result,
+                         _sys=sys,
+                         _get_thread_profile=_get_thread_profile,
+                         _get_thread_trace=_get_thread_trace):
+        # pylint:disable=unused-argument
+        _sys.setprofile(_get_thread_profile())
+        _sys.settrace(_get_thread_trace())
+
+    def _after_run_task(self, func, args, kwargs, thread_result, _sys=sys):
+        # pylint:disable=unused-argument
+        _sys.setprofile(None)
+        _sys.settrace(None)
+
     def __run_task(self, func, args, kwargs, thread_result):
+        self._before_run_task(func, args, kwargs, thread_result)
         try:
             thread_result.set(func(*args, **kwargs))
         except: # pylint:disable=bare-except
             thread_result.handle_error((self, func), self._exc_info())
         finally:
+            self._after_run_task(func, args, kwargs, thread_result)
             del func, args, kwargs, thread_result
 
     def run(self):
@@ -236,12 +262,23 @@ class ThreadPool(GroupMappingMixin):
     The `len` of instances of this class is the number of enqueued
     (unfinished) tasks.
 
+    Just before a task starts running in a worker thread,
+    the values of :func:`threading.setprofile` and :func:`threading.settrace`
+    are consulted. Any values there are installed in that thread for the duration
+    of the task (using :func:`sys.setprofile` and :func:`sys.settrace`, respectively).
+    (Because worker threads are long-lived and outlast any given task, this arrangement
+    lets the hook functions change between tasks, but does not let them see the
+    bookkeeping done by the worker thread itself.)
+
     .. caution:: Instances of this class are only true if they have
        unfinished tasks.
 
     .. versionchanged:: 1.5a3
        The undocumented ``apply_e`` function, deprecated since 1.1,
        was removed.
+    .. versionchanged:: NEXT
+       Install the profile and trace functions in the worker thread while
+       the worker thread is running the supplied task.
     """
 
     __slots__ = (
@@ -268,6 +305,8 @@ class ThreadPool(GroupMappingMixin):
         'task_queue',
     )
 
+    _WorkerGreenlet = _WorkerGreenlet
+
     def __init__(self, maxsize, hub=None):
         if hub is None:
             hub = get_hub()
@@ -437,7 +476,7 @@ class ThreadPool(GroupMappingMixin):
             self.manager = Greenlet.spawn(self._adjust_wait)
 
     def _add_thread(self):
-        _WorkerGreenlet(self)
+        self._WorkerGreenlet(self)
 
     def spawn(self, func, *args, **kwargs):
         """